1. Field of the Invention
The present invention relates to a liquid crystal light modulating material and, more particularly, to novel polymer dispersed liquid crystal for wide viewing angle.
2. Description of the Prior Art
In dependence on using a polarization screen, twisted nematic (TN) or super twisted nematic (STN) liquid crystals, chiefly used in conventional liquid crystal displays, have low use efficiency of light and need a surface orientation that becomes difficult around thin film transistors (TFTs) at high density of pixels. Furthermore, they have a small viewing angle range of .+-.20.degree. . To solve these problems, there have been recently proposed network liquid crystal (PNLC) of light scattering mode, which are liquid crystal materials dispersed in a polymeric matrix.
Incident light may be scattered when no electric field is applied by electrodes across the liquid crystal material, polymer-dispersed liquid crystal (PDLC), so that the cell comprising PDLC is opaque in appearance because the refractive indices of the liquid crystal and the polymer in PDLC are mismatched. The incident light may be transmitted through PDLC in the state of field-ON, that is, when an electric field is applied thereacross, so that cells comprising PDLC are transparent in appearance because the refractive indices of the liquid crystal and the polymer in PDLC are matched to allow PDLC to be aligned parallel to the electric field. This principle makes it possible to provide a switchable display employing no polarization screen. Though PDLC is superior to TN or STN in terms of the use efficiency of light and the viewing angle, the refractive index mismatching in PDLC may be excessive in the field-ON state, thereby giving rise to increasing haze and decreasing contrast, depending on view direction. Thus, for the purpose of actual application of PDLC to switchable windows or direct visible displays, it is required for the viewing angle to be widened.
A polarization screen, which has been used in a conventional liquid crystal display utilizing TN or STN liquid crystals exhibits low use efficiency of light and thus, contrast is lowered. Every effort has been made to apply to a visual display a method wherein a light transmission and scattering mode is utilized without using polarized light. Among these efforts, first issued was U.S. Pat. No. 4,435,047, which suggested a method in which liquid crystals were dispersed in transparent polymer. The method described in the above-mentioned patent comprises mixing liquid crystals with an aqueous solution of polyvinyl alcohol homogeneously to form an emulsion, casting the emulsion in a uniform thickness of 10 to 20 .mu.m on a glass slide or a polyester film which is pre-coated with indium tin oxide (ITO), a conductible material, vaporizing the water from the emulsion, and then laying a second polyester film or glass layer on the emulsion which has been cast on the first such that the emulsion is therebetween.
Another more developed method for the production of PDLC is described in U.S. Pat. Nos. 4,688,900 and 4,685,771. It is a phase separation method utilizing the solubility difference of polymer and liquid crystal monomer used in this method. As described, a matrix-providing composition containing liquid crystal, and monomer or oligomer of transparent resin, e.g., uncured epoxy, is polymerized by heat or UV light to yield microdroplets of liquid crystal in a thermoplastic or thermoset polymeric epoxy resin because as it polymerizes, the solubility of liquid crystal is decreased.
Assume that n.sub.s is the refractive index of transparent polymer resin, n.sub.o is the ordinary refractive index of a liquid crystal material, and n.sub.e is the extraordinary refractive index. When an electric field is not applied across the liquid crystal material, the refractive indices of n.sub.e and n.sub.s are not matched, as shown in FIG. 1A, so that incident light is scattered, and thus film sheets containing the liquid crystal material therebetween are opaque in appearance. On the other hand, as shown in FIG. 1B, when an electric field is applied, the refractive indices of n.sub.e and n.sub.s are matched so that the liquid crystal material is aligned parallel to the field. Accordingly, incident light is transmitted, and thus the film sheets appear transparent. Such principles result in the effect that the film seems to become widened, so as to be utilized as switchable windows, and that the film allows a screen constituted thereby to be brightened, so as to be utilized as a projection display or a direct visible display. Also, the liquid crystal materials which are utilized according to such principles are superior in viewing angle and response time to TN or STN liquid crystals. The viewing angle, however, is restricted to .+-.45.degree. because in the state of field-ON, the structural difference of the polymer and the liquid crystal in PDLC makes the n.sub.s and n.sub.o for incident light mismatched at an oblique angle.
In PNLC, polymeric matrix has a structure of three-dimensional crosslinked network and liquid crystal has a continuous phase, whereas in PDLC, polymeric matrix has a continuous phase and liquid crystal is formed into droplets. PNLC is less dependent on the refractive indices of polymeric matrix and liquid crystal and has a wider viewing angle than PDLC, the operative principle of which is different from that of PNLC in the aspect of the dependence on the refractive indices (Japanese Patent Laid-Open No. Hei. 01-198725). Even PNLC has a limitation of viewing angle because transparency decreases at increasing oblique angles, giving rise to increasing haze until an essentially opaque appearance is detected at an oblique-enough angle.
As shown in FIG. 2, a process for the production of polymer-dispersed liquid crystal material which comprises an anisotropic, birefringent polymeric matrix having units attached as pendants to the main chain of the polymer, the structure of which is similar to that of liquid crystal monomer, is disclosed in International Patent No. WO89/09807 (incorporated herein by reference). The polymer-dispersed liquid crystal material is made in such a manner that the ordinary refractive index n.sub.o ' and the extraordinary refractive index n.sub.e ' of the liquid crystal are selected to have values identical to the ordinary refractive index n.sub.o ' and the extraordinary refractive index n.sub.e ' of the polymer, respectively. This allows the refractive indices of the polymer and the liquid crystal to be identical at all viewing angles in the presence of an applied electric field so as to widen the effective viewing angle. The birefringent polymeric matrix, however, is difficult to synthesize in a manner as mentioned above. Moreover, the polymeric matrix is compatible with the liquid crystal monomers well, so that the phase separation of the polymeric matrix from the liquid crystal monomers may not be carded out easily, and the viewing angle becomes narrow in the presence of various liquid crystal monomers which have different structures from one another.
A polymer-dispersed liquid crystal material which had wide viewing angle by making use of the focal conic texture and bi-stability of cholesteric liquid crystal was reported by Institute of Electrical and Electronics Engineers, 1991. It employs a helix structure which is constructed when the cholesteric liquid crystal is placed between two glass sheets bearing indium tin oxide, rubbed with rubbing material, e.g., polyimide or the like. By this orientation treatment, the pitch of the helix is thus controlled to be identical to a wavelength of infrared ray range, so that reflection or scattering does not occur at the range of visible light wavelength. In the absence of an applied electric field, the polymer-dispersed liquid crystal materials are textured to be planar due to the effect of surface orientation according to the helix structure, so as to be transmissive to incident light whereas in the presence of a field, the effect disappears to make the polymer dispersed liquid crystal materials have focal conic textures, which is maximized in entropy, so that incident light is scattered. However, there are disadvantages of a poor visual contrast and requirements of a high operating voltage and an orientation treatment.